Strain rate sensitivity and activation volume of AISI 321 stainless steel under dynamic impact loading: Grain size effect

Highlights: • Strain rate sensitivity (m) and activation volume (ϑ^∗) of AISI 321 steel were studied at high strain-rate regime. • m and ϑ^∗ were determined for UFG (0.24 μm), FG (3 μm) and CG (37 μm) samples. • m and ϑ^∗ remains unchanged for UFG at all investigated strain rates. • A change in both m and ϑ^∗ occurred at critical true strain rates (${\dot{\epsilon }}_{\mathrm{Tc}}$) of ~5900 and 6800 s⁻¹ for FG and CG specimens, respectively. • The change in ${\dot{\epsilon }}_{\mathrm{Tc}}$ with increasing grain size is attributed to complexities in the operating deformation mechanisms in AISI 321 steel. -- Abstract: The rapid change in strain rate sensitivity (SRS) of metals at a critical true strain rate (${\dot{\epsilon }}_{\mathrm{Tc}}$) is commonly attributed to change in rate-controlling deformation mechanism from thermally-activated process to viscous drag. In this study, metastable AISI 321 stainless steel with grain sizes of 0.24 μm (ultrafine [G1]), 3 μm (fine [G2]) and 37 μm (coarse [G3]) was deformed at high strain-rate. A change in SRS with increasing ${\dot{\epsilon }}_T$ is observed only in G2 and G3 and it occurs at ${\dot{\epsilon }}_{\mathrm{Tc}}$ of ~5900 and 6800s⁻¹, respectively; suggesting that grain size have an effect on the switch of rate-controlling deformation mechanisms. While the SRS of G1 is 0.101 and unchanged, those of G2 and G3 change from 0.094 to 0.326 and 0.091 to 0.634, respectively, once ${\dot{\epsilon }}_{\mathrm{Tc}}$ is reached. Similarly, the estimated activation volume in G1 specimen is ~1.57b³, while those of G2 and G3 specimens change from ~2.95b³ to ~0.65b³ and from ~4.10b³ to ~0.45b³, respectively. Analysis of the deformed specimens using XRD, EBSD and TEM techniques revealed that the complexities in the activated deformation mechanisms in G2 and G3 specimens led to a change in ${\dot{\epsilon }}_{\mathrm{Tc}}$. Constant SRS in G1 specimen indicated no change in the operational deformation mechanisms.